In a wireless communication system, a base station communicates with wireless devices in a plurality of cells, each cell corresponding to a particular geographic area. Some or all of these cells may be activated at the same time. Each cell is associated with one or more antennas that radiate into the geographic coverage area of the cell to establish and maintain communications with the wireless devices in the cell.
FIG. 1 is a block diagram of one path 10 in a base station for processing and transmitting downlink symbols to a cell coverage area. A processing unit 12 forms symbols of information and sends these symbols to an antenna mapper 14. Each cell is associated with an antenna mapper 14 which receives and stores symbols to be transmitted into the cell coverage area. Each symbol is written to a memory location in a transmission buffer memory 16. An inverse Fast Fourier Transformer, (iFFT), 18 reads a symbol from the transmission buffer memory 16, transforms the symbol and forwards the transformed symbol to a radio transmitter 20 which has a power amplifier and an antenna to amplify and transmit the symbol to a cell coverage area. The process of writing a symbol from the antenna mapper 14 to the transmission buffer memory 16 is synchronized by a timing source 22 with the reading of a symbol from the transmission buffer memory 16 by the iFFT 18, and is also synchronized with transmission by the radio transmitter 20.
As modern deployments of base station have 24, 48 or higher numbers of cells, the amount of transmission buffer memory required becomes a substantial cost. Further, many systems may store two TTI buffers per symbol to create a “ping pong” scheme, thereby doubling the required memory. Also, as cells of a base station are dynamically deployed without shutting down the base station, fragmentation of memory results.
An example of a buffer reallocation problem due to fragmentation is explained with reference to FIGS. 2-5. FIG. 2 is a block diagram of a configuration at a base station in which 6 cells are activated in an initial cell deployment at a time to. Note that in a typical implementation, there may be many more cells, for example, 20 to 50 cells. In the configuration of FIG. 2, each cell has a bandwidth of 10 Mega-Hertz (MHz). In other deployments, different cells may have different bandwidths. Each cell has its own antenna mapper 14a-f, referred to herein collectively as antenna mappers 14. Each mapper 14 writes symbols to a different memory segment 16a-f of the transmission buffer memory 16. The size of an address space of a memory segment to which a symbol of a cell is written is proportional to the bandwidth of the cell.
Each memory segment 16a-f is read by a corresponding one of iFFTs 18a-f, referred to herein collectively as iFFTs 18. Each memory segment 16a-f is a circular memory, which means that a memory segment is written to progressively from a first memory address to a last memory address of the memory segment, and when the last memory address is written to, the writing starts anew at the first memory address of the memory segment. Each iFFT 18a-f is coupled to a respective radio 20a-f, referred to herein collectively as radios 20. The reading and writing for each memory segment 16a-f is synchronized by symbol timer signals from the timing source 22, so that a writing to one address of a memory segment coincides with a reading from another address of that same memory segment.
FIG. 3 shows a deployment of the base station at a time t1 subsequent to, where cells 0, 1, 2, and 4 have been removed, leaving only cells 3 and 5. Removal of cells may be the result of reconfiguration of the base station owing to the desire to reallocate bandwidth among supported cells. Memory segments 16a-16c and 16e are now free. FIG. 4 shows a subsequent deployment at time t2 where a new cell 0 having a bandwidth of 20 MHz is added, and utilizes memory segments 16a and 16b, each corresponding to a 10 MHz cell, of the transmission buffer memory 16. FIG. 5 shows an attempted deployment at a subsequent time t3 of a new cell 1 having a 20 MHz bandwidth. Even though there enough total buffer memory to support a 20 Mhz cell, the redeployment fails because there is not 20 MHz of contiguous memory remaining in the transmission buffer memory 16. As is shown, there are two discontiguous portions that total the amount of buffer memory required. In other words, the memory is fragmented. Allocation of memory segments 16c and 16e to cell 1 would require memory de-allocation and lead to complex memory management requirements, or an undesirable reboot of the base station would need to be made to rearrange the memory allocations. Operators do not like to have to reboot base stations as such an action disconnects users calls, delays reconnection, and may adversely impact operators who have no relationship to the reallocation issue.